<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .outerDiv {
            width: 400px;
            height: 300px;
            border: 1px solid red;
        }
        .innerDiv {
            width: 100px;
            height: 100px;
            border: 1px solid yellow;
        }
        .d1 {
            float: left;
        }
    </style>
</head>
<body>
    <div class="outerDiv">
        <div class="innerDiv d1">1</div>
        <div class="innerDiv d1">2</div>
        <div class="innerDiv d1">3</div>
    </div>
</body>
</html>